Both you and the mother have good points; children on the autistic spectrum do have a more difficult time with transitions than most children, and the approach of "transition time" can definitely reduce the stress of these children, but "learning how the real world works" would be beneficial to the children in the long term.
Perhaps some sort of medium could be reached; would it be possible to start a child off on 15 minutes of "transition time," then, maybe after two or three times, if the child seems to be doing well, 10 minutes of transition time, and, if the child continues to do well, gradually keep lessening the transition time until no warning is given at all?
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I am not a textbook case of any particular disorder; I am an abstract, poetic portrayal of neurovariance with which much artistic license was taken.